Drying is used in a variety of industries such as agricultural, marine products, timber, textile, chemical, paper, etc. Drying is typically performed by keeping the products at a high temperature, approximately 60° to 80° C., for a long time. Drying is an energy-intensive process and optimization of systems and processes to achieve more cost-effective drying processes will result in significant reduction of energy consumption, which may provide considerable cost savings.
In conventional systems for drying, i.e. kilning apparatus, the drying air is directly heated by using oil or gas burners. Thus, in addition to reducing the energy consumption in drying systems, there is also a desire that such systems should be environmentally friendly, with no or minimal emissions of combustion products originating from the burning of fossil fuels.
Other conventional drying systems comprise dehumidifiers, heaters and heat exchangers. U.S. Pat. No. 4,532,720 relates to a drying process and apparatus utilizing a refrigeration cycle. The drying apparatus is constructed to interpose a heat exchanger between an evaporator and a heater so that air in a drying chamber may be dried and heated in a cycle of introducing hot air in the drying chamber into the heat exchanger to pre-cool hot air, guiding pre-cooled air to the evaporator to dry it due to dehumidification by cooling, introducing cooled and dried air into the heat exchanger again to increase the temperature, introducing the air to the heater to further heat it and returning dried and heated air to the interior of the drying chamber. The heater is a condenser and an electric heater, providing hot air to the drying chamber keeping the drying chamber at a high temperature of about 60-80° C. In the system described in U.S. Pat. No. 4,532,720 the air from the drying chamber is recycled to the dehumidifier in a closed circulation system. This is a normal design for such drying apparatus.
The growing demand for renewable energy sources has led to an increased focus on bioenergy resources. The use of forest resources for bioenergy is therefore expected to increase considerably. The prevailing use of biofuel is the direct combustion of wood, wood chips and pellets for heating in stationary plants, either in conventional wood stoves, central heating systems or remote heating plants. Different wood chips heating installations require different moisture content in the wood. Some heating installations have optimal efficiency at moisture content below 30%, while others may require 40-50%. Dry wood chips will give a complete combustion with lesser emissions and provide better effect on the boiler. Trees on root normally have a moisture content of approximately 50% of the total weight, meaning half the weight of the tree is water and the rest is dry matter. Thus, the use of forest resources as bioenergy involves drying and intermediate storing of the energy wood chips.
The European standard series EN 15234, “Solid biofuels—Fuel quality assurance” is provided as general requirements and additional product standards, and consists of 6 Parts. Part 4: Wood chips for non-industrial use (EN 15234-4:2012) covers the raw material supply, production and delivery chain. The said part defines inter alia dimensions for wood chips, specifying P-classes defining particle sizes, size fractions and dimensions, e.g. in class P16 the main fraction (at least 75% of the total mass) must be between 3.15 and 16 mm. Particles smaller than 3.15 are defined as fines fraction and this amount must be less than 12 weigh-%. The sizes/dimensions for the coarse fraction are also defined. Other P-classes are P31.5: at least 75 weight-% is between 8-31.5 mm; P45: at least 75 weight-% is between 8-45 mm; and P65: at least 75 weight-% is between 8-63 mm.
M. Heling (in Helin, M., “Wood as a fuel & drying of wood chips”, 2005, North Karelia Polytechnic, Finland) gives an example which illustrates the effect of correct moisture in the wood chips. The amount of heat out of 1 loose-m3 of wood chips with two different moisture contents using a 50 kW boiler:
Moisture content 50%, net heating value≈800 kWh/loose-m3.
The boiler efficiency at 50% moisture is≈62%.
The available energy (heat) is 0.62×800 kWh≈500 kWh.
Moisture content 30%, net heating value≈890 kWh/loose-m3.
The boiler efficiency at 30% moisture is≈78%.
The available energy (heat) is 0.78×890 kWh≈700 kWh
Using wood chips with 30% moisture instead of 50% moisture will thus result in 40% more heating energy in a 50 kW boiler.
Most small-scale and medium sized wood chips heating installations have optimal combustion at moisture content of 20-25%. In production of pellets and wood powder for gasification the requirement is 10-12% moisture. Traditional drying of wood chips involves storing logs for drying in cold open air or covered until moisture content in the logs is about 30%, before wood-chipping. The time for such drying is however very long. In general, the moisture content in storage stable wood chips should be less than about 25% since higher moisture content will increase the risk of heat generation in the stored wood chips, and in the worst case spontaneous ignition. The cost for artificial drying may be high compared to the gain of energy in the dried wood chips. Thus, there will be a positive energy balance only if the energy used for artificial drying the wood chips is less than the resulting energy content in the wood chips. Economical methods for drying wood chips may involve usage of waste heat from the process industries, incinerations plants or power plants. However, when there is no such waste heat available, the drying of wood chips must be carried out by lowest possible consumption of supplied energy in the form of electrical power or other energy source.
With reference to the prior art drying systems there is a desire to provide a drying system and a drying method that significantly reduces the energy consumption, shortens the time for the drying process and results in uniform drying of the products to be dried, compared to conventional drying systems. Such drying system should be equally suitable for drying different kinds of materials and products, generally divided in loose bulk materials and single piece products. Further, the drying system should be suitable for drying products which are temperature sensitive, which is often the case when drying food products. Some traditionally prepared food products is dependent on a certain climate, e.g. for drying fish the temperature must neither be too low or too high. The recent climate changes leads to challenges for such traditional methods. Thus, there is also a desire for a drying system which can be used in the production of such food products.
The present inventors found that when utilizing the heat (energy) at both the cold and the warm side of a heat pump, while controlling the air conditions throughout the drying system, very good results are achievable with regards to the above said desired properties. The inventors surprisingly found that providing dry, only moderately heated air into a drying chamber resulted in a very effective drying process both in reducing the drying time and in energy consumption per weight of dried product. Keeping the drying temperature in the drying chamber at a moderate level, i.e. 20-35° C., means that all required heat may be provided by a conventional (air-to-air type), e.g. heat pump for domestic use, or in larger drying apparatus a large industrial air-to-air heat pump.
A heat pump provides heat in a very efficient way compared to using the same amount of power by heating with a panel (electrical) heater. The performance of a heat pump is given by the term, coefficient of performance (COP), which is used to describe the ratio of useful heat movement per work input. At outside temperature of e.g. 10° C. a typical air-source heat pump has a COP of 3 to 4, at optimal conditions a COP of 5 may be achieved.
The present inventors found that by passing a slightly heated, dry airflow through or past moist material a large amount of humidity is taken up by the drying air. By passing the air leaving the drying chamber through a heat exchanger in thermal communication with cold, dry air flowing into the drying system, it is possible to recover most of the heat. The present inventors found that by combining these findings in an optimal system, wherein the air atmosphere and pressure are controlled, it is possible to attain a drying system which utilizes the supplied energy optimally, resulting in a very cost effective method for drying material. Using the drying system according to the present invention resulted in a reduction 50% less energy consumption (power consumption) compared to a conventional “modern” dryer which recycles the drying air to recover heat. Also, the drying system according to the present invention reduces the drying time by 50% compared to the said conventional modern dryer. Compared to other conventional dryers (without recycling drying air) resulted in up to 75% reduction in power consumption. Thus the present invention results in considerable savings in costs, energy consumption and time. It also has the possibilities of scaling up or down with regards to the amounts of materials to be dried. Another advantage of the present invention in relation to known drying system is that the present system eliminates the need for a separate air dehumidifier. The drying system according to the present invention also eliminates the need of an extra heater, as the heat demand is covered by the heat pump.
The present drying system can be used for drying loose bulk materials and single piece materials/products. In this context, loose bulk material refers to materials in granular or particular form or a mass of relatively small solids. Examples of loose bulk materials are grains (wheat, malt, maize, rice, barley, oats, rye, sorghum, soybeans, etc.), wood chips, and agricultural products like fruits which must be dried during processing. Further, the term “single piece materials/products” should be understood as products/materials which are not considered bulk materials, but handled individually, or only a few at the time. Examples of such products are fish products, meat products, vegetables or fruits, wood products, marine products, etc.
In this context the expressions “drying”, “dehumidifying” and “lowering the moisture” are used interchangeably and should have the same meaning unless explicitly otherwise stated.